A known autostereoscopic display device comprises a two-dimensional liquid crystal display panel having a row and column array of display pixels (wherein a “pixel” typically comprises a set of “sub-pixels”, and a “sub-pixel” is the smallest individually addressable, single-color, picture element) acting as an image forming means to produce a display. An array of elongated lenses extending parallel to one another overlies the display pixel array and acts as a view forming means. These are known as “lenticular lenses”. Outputs from the display pixels are projected through these lenticular lenses, whose function is to modify the directions of the outputs.
The pixel comprises the smallest set of sub-pixels which can be addressed to produce all possible colors. For the purposes of this description, a “unit cell” is also defined. The unit cell is defined as the smallest set of sub-pixels which repeat to form the full sub-pixel pattern. The unit cell may be the same arrangement of sub-pixels as a pixel. However, the unit cell may include more sub-pixels than a pixel. This is the case if there are pixels with different orientations of sub-pixels, for example. The overall sub-pixel pattern then repeats with a larger basic unit (the unit cell) than a pixel.
The lenticular lenses are provided as a sheet of lens elements, each of which comprises an elongate partially-cylindrical (e.g. semi-cylindrical) lens element. The lenticular lenses extend in the column direction of the display panel, with each lenticular lens overlying a respective group of two or more adjacent columns of display sub-pixels.
Each lenticular lens can be associated with two columns of display sub-pixels to enable a user to observe a single stereoscopic image. Instead, each lenticular lens can be associated with a group of three or more adjacent display sub-pixels in the row direction. Corresponding columns of display sub-pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right a series of successive, different, stereoscopic views are observed creating, for example, a look-around impression.
FIG. 1 is a schematic perspective view of a known direct view autostereoscopic display device 1. The known device 1 comprises a liquid crystal display panel 3 of the active matrix type that acts as a spatial light modulator to produce the display.
The display panel 3 has an orthogonal array of rows and columns of display sub-pixels 5. For the sake of clarity, only a small number of display sub-pixels 5 are shown in the Figure. In practice, the display panel 3 might comprise about one thousand rows and several thousand columns of display sub-pixels 5. In a black and white display panel a sub-pixel in fact constitutes a full pixel. In a color display a sub-pixel is one color component of a full color pixel. The full color pixel, according to general terminology comprises all sub-pixels necessary for creating all colors of a smallest image part displayed. Thus, e.g. a full color pixel may have red (R) green (G) and blue (B) sub-pixels possibly augmented with a white sub-pixel or with one or more other elementary colored sub-pixels. The structure of the liquid crystal display panel 3 is entirely conventional. In particular, the panel 3 comprises a pair of spaced transparent glass substrates, between which an aligned twisted nematic or other liquid crystal material is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces. Polarizing layers are also provided on the outer surfaces of the substrates.
Each display sub-pixel 5 comprises opposing electrodes on the substrates, with the intervening liquid crystal material there between. The shape and layout of the display sub-pixels 5 are determined by the shape and layout of the electrodes. The display sub-pixels 5 are regularly spaced from one another by gaps.
Each display sub-pixel 5 is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display by providing addressing signals to the switching elements, and suitable addressing schemes will be known to those skilled in the art.
The display panel 3 is illuminated by a light source 7 comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source 7 is directed through the display panel 3, with the individual display sub-pixels 5 being driven to modulate the light and produce the display.
The display device 1 also comprises a lenticular sheet 9, arranged over the display side of the display panel 3, which performs a light directing function and thus a view forming function. The lenticular sheet 9 comprises a row of lenticular elements 11 extending parallel to one another, of which only one is shown with exaggerated dimensions for the sake of clarity.
The lenticular elements 11 are in the form of convex cylindrical lenses each having an elongate axis 12 extending perpendicular to the cylindrical curvature of the element, and each element acts as a light output directing means to provide different images, or views, from the display panel 3 to the eyes of a user positioned in front of the display device 1.
The display device has a controller 13 which controls the backlight and the display panel.
The autostereoscopic display device 1 shown in FIG. 1 is capable of providing several different perspective views in different directions, i.e. it is able to direct the pixel output to different spatial positions within the field of view of the display device. In particular, each lenticular element 11 overlies a small group of display sub-pixels 5 in each row, where, in the current example, a row extends perpendicular to the elongate axis of the lenticular element 11. The lenticular element 11 projects the output of each display sub-pixel 5 of a group in a different direction, so as to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.
The skilled person will appreciate that a light polarizing means must be used in conjunction with the above described array, since the liquid crystal material is birefringent, with the refractive index switching only applying to light of a particular polarization. The light polarizing means may be provided as part of the display panel or the view forming arrangement of the device.
FIG. 2 shows the principle of operation of a lenticular type view forming arrangement as described above and shows the light source 7, display panel 3 and the lenticular sheet 9. The arrangement provides three views each projected in different directions. Each sub-pixel of the display panel 3 is driven with information for one specific view.
In the designs above, the backlight generates a static output, and all view direction is carried out by the lenticular arrangement, which provides a spatial multiplexing approach. A similar approach is achieved using a parallax barrier.
The lenticular arrangement only provides an autostereoscopic effect with one particular orientation of the display. However, many hand held devices are rotatable between portrait and landscape viewing modes. Thus, a fixed lenticular arrangement does not allow an autostereoscopic viewing effect in different viewing modes. Future 3D displays, especially for tablets, mobile phones and other portable devices will thus have a possibility to observe 3D images from many directions and for different screen orientations. Modern LCD and OLED display panels with existing pixel designs are not suited for this application. This issue has been recognized, and there are various solutions.
A dynamic solution involves providing a switchable lens arrangement, which can be switched between different modes to activate the view forming effect in different orientations. There may essentially be two lenticular arrangements, with one acting in pass through mode and the other acting in lensing mode. The mode for each lenticular arrangement may be controlled by switching the lenticular arrangement itself (for example using an LC switchable lens array) or by controlling a polarization of the light incident on the lenticular arrangement.
A static solution involves designing a lens arrangement which functions in the different orientations. A simple example can combine a rectangular grid of square sub-pixels in the display with a rectangular grid of microlenses (where the lens grid directions are either slanted or non-slanted with respect to the pixel grid directions) to create multiple views in both display orientations. The sub-pixel shapes should be preferably close to a 1:1 aspect ratio, as this will allow avoiding a problem of different angular width for individual views in portrait/landscape orientations.
One possible disadvantage of this approach is a banding effect, in which the black matrix areas between the sub-pixels are projected to the viewer as a regular pattern. Partially it can be solved by slanting the lens array. Specifically, in order to reduce banding effect due to projection of periodic black pixel matrix a view forming arrangement need to be chosen with respect to the pixel addressing direction (rows/columns).